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MAINTENANCE AND LIFE TIME EXTENSION OF

POWER TRANSFORMERS

Kari Heinonen

Infratek Finland Oy

2010 EuroDoble Colloquium

18-20 October 2010

Birmingham, United Kingdom

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10.9.2010 K Heinonen, Infratek Finland Oy 2

ABSTRACT Visual inspections

Oil testing

Oil filtering

Bushings

OLTC

Transformer refurbishment

Copper corrosion

Failure investigation

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Fig. 1. Finnish 110 kV 400 kV transformer population.

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Vuosimalli

k

pl

Perushuolletut Huoltamattomat

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1 Visual inspections

1 to 6 monthsinterval

Bushings:Oil level, Leakages

etc.

Corrective actionsimmediately withdangerous findings

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2 Oil analysis

There are not any

excuses why not to applyfrequent oil analysing.

DGA and break

down voltage test:2 years period

Otheranalysis:

10 year period

Warranty period: morefrequent monitoring

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2 Oil analysis continues

Bushings with oil-

paper insulation: DGA10 years interval, OR

Power Factor

measurement, 10

years interval

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FIELD TESTING RESULTS, Bushing

ASEA GOB 650, 123 kV, 800 A

Ser. Numb. PF1 PF2 C1 C2

104807 0,14 % 80,5 % 180 pF 190 pF

IEC -60137 < 0,7 %

DGA results ppm (KELMAN Transport X)

H2 CO2 CO C2H4 C2H6 CH4 C2H2 TCG

27 3819 964 4 9 18

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BUSHINGS 110 kV DGA results ppm (KELMAN Transport X and Lab.analysis)

Serial

TYPE Numb H2 CO2 CO C2H4 C2H6 CH4 C2H2 TCG

GOB 650 104807 27 3819 964 4 9 18

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3 On-site and on-line gas analysis

280 MVA,

400/15 kV

On-line gas

monitoring

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4 Oil filtering, degassing and regeneration

On-site oil filtering: Start ofrepair activities needs to bedelayed

Not any corrective action

Low break down voltage:Filtering and oil degassing

Continuous degassing

Oil regeneration: Aged oil asgood as new oil.

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5 Monitoring of transformer bushings

Oil sampling or powerfactor measurement 10year period

When refurbishingTrafo:Lab. Testing: voltage test,tan delta, partial

discharges, capacitance.

2,0 % of all 110 kVbushings tested had minorfailures and 2,5 % ofbushings had to bescrapped immediately

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5 Monitoring of transformer bushings continue

LAB. TESTING RESULTS

ASEA GOB 550, 123 kV, 800 A

PD PD

Ser. Numb. Tan d 0,61 x Un 1,0 x Un.

141399 0,45 % < 3 pC < 3 pC141341 0,46 % < 3 pC < 3 pC

141334 0,45 % < 3 pC < 3 pC

141335 0,50 % < 3 pC < 3 pC

IEC -60137 < 0,7 % < 5 pC < 10 pC

Voltage test 0,75 x 230 kV, 50 Hz, 1 min

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6 On-load tap-changer service

Five to six years

period

Oil sampling, visual

inspections, relaytesting, bushing and

transformer power

factor testing etc.during the same

outage.

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7 Transformer refurbishment

Refurbishment: Lifting of active part

(core and windings; untanking)

On-site, power plant, repair shop,

at the manufacturer

Finland ca. 30 % of all 110 kV

400 kV transformers(ca. 2000 pcs.) revised

WHY: Better reliability,

Longer lifetime, Environment

Cost 1520 % of new price.

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7 Transformer refurbishment continues

110 kV transformers:

Trailertransportationto workshop

4 to 6 weeks including

transportations

On-site refurbishment:

* 110 kV, 100 MVA +

* (220 kV transformers)

* 400 kV transformers

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Tightening of windings

Drying of paper insulation

Revision and modifications oftap-changerincludingselector

Oil degassing and filtering

High-voltage testing ofbushings Replacement of gaskets

Replacement of protection relays and devices

Addition of a separate expansion vessel for on-load

tap-changer etc. Painting

Documentation

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.


Findings 1 / 120 kV, 63 MVA, 1974

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.


Findings 2 / 121 kV, 50 MVA, 1969

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8 Refurbishment of distribution andspecial transformers

50 kVA 1000 kVA: Whenmoving the transformer

Electric filter rectifiertransformers: Oil samplingwith five years interval andrevision by lifting the active

part with ten years interval.

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9 Other maintenance methods

Thermocamera:

Local surface hot-spots, loose contacts

Low voltagetransformer busbars:plastic coatingagainst animals

Replacement of oldtype surge arresters

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10 Copper corrosion

Some oils are

more risky thanthe others.

Identify risky trafos

Start actions toreduce the risk

(lower temperature,passivator addition).

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11 Failure investigations

Data: protectionrelays, fault recorders,visual and audible

observations etc..

Electricalmeasurements andoil testing

False alarm?

Can we repair it on-site?

Active part lifting isneeded?

Transport is needed?

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11 Failure investigations continuesFAILURE STUDY CHECK LIST

Date and time of failure (hour, minutes and seconds, if also seconds are available))

Which protection relays / which functions have activated and how (alarm or/and tripping; gas relaytripping before alarm etc.)?

Take the oil sample for laboratory analysis (bottom valve) immediately after the failure and about 24hours after the failure. For 220 and 400 kV transformers also top valve sampling immediately after the

failure. Is there gas in gas relay, how much, what color, is it air or flammable gas? Take gas sample also for

laboratory analysis.

What indications do fault recorders have related to tripping (also nearby substations)?

Transformer load when tripping and mean load 1 hour and 24 hour prior tripping?

On-load tap-changer (OLTC) position when tripping?

Has OLTC operated during tripping and to which direction?

OLTC counter reading and operations from previous OLTC-revision?

Temperature indicators (oil, winding) indication: indication when tripping and max. pointer indications? All abnormal findings according to visual inspection (tank or/and radiator deformations etc.)?

Marks of arching: Bushings, transformer cover, cables etc.?

Cleanness of porcelain insulators?

Dead animals and otherdebris on transformer cover?

Abnormal sounds from transformer before failure?

Has there been breaker or disconnector operations near the transformer before tripping (same or

nearby substations)? Have there been transmission or distribution network disturbances or over voltages just before orduring tripping?

Weatherconditions during and before tripping?

Is there some abnormal according to visual inspection with surge arresters, protection capacitors,disconnectors, circuit breakers and other equipment near the transformer?

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12 Conclusions

Utilize the existing transformer capacity as long time as

possible

There are not any excuses why not to apply frequent oil

analysing

Bushings shall be oil sampled or power factor

measured every 10th year

Transformerrefurbishment / untanking Once during

lifetime

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Thank you foryour attention

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